28 thoughts on “Recovering Falcon Second Stages”

    2. 3 pieces? That’s probably just the combustion chamber shown. Heck even a regen cooled nozzle can have more pieces than that. I doubt you could make a turbopump with so few pieces.You will want some kind of pump for anything that reaches orbit, otherwise you lose a lot of performance.

  2. Interesting that he focused so much on the sea landings for S1. I would think that in the long run almost all of them would be landing back at the Cape, as well as (after significant at-sea test landings) the S2s.

    1. Only missions with fairly light payloads and to fairly low-altitude orbits leave enough propellant in the S1 tanks to allow the boostback maneuver needed to land on land. That is a minority of F9 missions at this point.

      It’s quite possible SpaceX will pick up a lot of future business putting up bunches of smallsats per mission during initial deployments. Many of these missions might allow RTLS and landings on land. This would certainly help SpaceX’s overall logistics situation.

  3. Imagine an entry shroud that could be fitted over a S2 in orbit. The S2 would just have to be able to maneuver to an orbit where a stockpile of these exist. The shrouds themselves could be design to nest together for a separate bulk launch.

    1. Interesting notion. Nicely sidesteps most of the parasitic mass problem, though the S2’s would still need to be able to get to whatever altitude and inclination was chosen for the parking orbit of the recovery module dispenser.

      Even more interesting if the recovery modules were, themselves, reusable. Maybe do an occasional FH mission just to put up a batch of these as they come off the line or after they’ve been spruced up following use? Kind of like reloading the plate dispenser in a cafeteria.

  4. I think it’s less complicated that what they said over there. SpaceX could simply wait until they have a mission where they can land the first stage at the Cape. Then they can use the drone ship to recover the second stage. Plus, like they said, the first flights could simply do a hoverslam into the ocean until they figure out how to do the recovery right.

    Still this is going to be a lot more complicated than first stage recovery. The reentry speeds are much higher, the thermals much worse, and it requires much deeper engine throttling since you have a single engine. The required extra fuel margin woudln’t be an issue in the Falcon Heavy test flight. So I think that flight would be the most appropriate to begin testing second stage recovery.

    1. If most F9 missions allowed S1 recovery to land, you’d be right. But, at least for awhile, such missions will be heavily outnumbered by those requiring too much S1 propellant use to allow a later boostback maneuver.

  5. I don’t think the S2 recovery design will include landing legs, or propulsive touch down.

    Keep in mind that every pound devoted to recovery is directly subtracted from payload. There is a very high incentive to minimize recovery mass TAKEN TO ORBIT on S2.

    At the very minimum, you need to:

    * Deorbit :: ( $x pounds of extra propellent)
    * Survive re-entry :: (heat shield and/or propellent )

    Now the recovery near the ground can be other assets. I hypothesize that the second stage will have a small parachute and be caught by a plane/helicopter. This is possible as the second stage can de-orbit most places on earth.

    Contrary to the article, this will be on land somewhere. No reason to do this at sea. The stardust mission gives a precedent for de-orbiting into the continental US.

    1. The ideal place to de-orbit and recover the second stage would be Jeff Bezos’ launch facilities. If I recall correctly, under the OST Bezos couldn’t touch them, so Elon could tweet: “Swinging by your place to pick up my rocket THAT HAS BEEN IN ORBIT.”

      1. Blue Origin and SpaceX are both American companies, so the OST would not apply. SpaceX would either have to come to some agreement with BO or they’d be trespassing.

        1. Hrm… Elon would have to change tweets then.

          Elon: “Thought you’d like to test your facilities with a rocket THAT’S ACTUALLY BEEN IN EARTH ORBIT.”

          Jeff: “Why does it look so toasted?”

          Elon: “Because it flew beyond Culberson County’s airspace. You should try that sometime. Also, your capsule looks like a GMC Pacer.”

          1. Hmmm,,,, intentionally crash landing at the other guys launch facility? Sounds like a 70s movie plot.

    2. There’s far less of an incentive if you’re launching a six-ton satellite to GTO on a Falcon Heavy. I’d imagine they could spare a few tons to recover the second stage, and still have enough excess capacity to recover all three boosters.

      And, if they can do that, Falcon 9 may be demoted to only launching small payloads where it can also have enough excess capacity to recover the second stage.

    3. Can’t entirely rule out your scenario. But the Stardust lander was miniscule compared to an F9 S2. And snagging something as big as an F9 S2 in mid-air has never been done. The FAA might not be too eager to risk crashing things that size even in remote areas of the U.S. western deserts.

  6. I think the most likely candidates for S2 recovery will be on LEO missions due to the lower reentry speeds. Entries from GTO will be going much faster and more difficult, at least at first. SpaceX might go with an inflatable heat shield or blunt shield on the top end of the stage. The stage already has hypergolic thrusters but they will need more fuel capacity, as will the stage’s batteries which reportedly are only good for about 5 hours.

    Landing using the vacuum-optimized Merlin seems unlikely because even at its lowest throttle settings, it produces too much thrust plus the nozzle is much too large for low altitude operations.. I think a good option would be 2 or 3 SuperDraco thrusters mounted at the top of the stage firing towards the bottom. These thrusters are available, throttleable, and have about the right amount of thrust for an empty stage. Landing legs would have to be pretty long to extend past the Merlin engine bell.

    I’d still use grid fins to provide additional drag and control during the descent. That will save propellant for landing. Instead of trying to come back to the Cape, I’d go for a landing off the coast of California. By the time S2 reaches orbital velocity, it’s going too fast and is much too far downrange to come back to the Cape, while trying to land there after completing most of an Earth revolution would require overflying a lot of populated land. It seems better to have most of the sonic booms and risk over the Pacific Ocean.

    1. – Mass distribution makes the stage want to reenter engine first. Nose first should be possible, but would need a very active control system. As for protecting an engine during reentry, I’d like to see how passing a coolant through the engine and out the nozzle works with an oncoming hypersonic airstream.

      – Another reason recovery from LEO will be easier than from GTO: From LEO you can wait a few orbits until your flight path takes you close to the launch facility. Returning from GTO you don’t have nearly as much choice of where you hit reentry, unless you expend delta-V or aerobreak into LEO first.

    3. The battery life issue certainly needs addressing. Some recovered S2’s might have to spend days in orbit before coming down. Maybe fitting the S2 with conformal solar panels would do the trick. Mr. Musk is alleged to know a thing or two about both batteries and solar panels.

      Throttleability of the Merlin Vac may be subject to considerable improvement without much engineering effort. The Apollo LM descent engine could throttle down to 10%. It was a pintle-injector engine as is the Merlin Vac.

      The engine bell issue remains. SpaceX could go to a partially retractable design at some penalty in extra mass. Perhaps said penalty could be offset with reduced mass for shorter landing legs.

      Agree that at-sea S2 recoveries will probably be de rigeur owing to both safety and NIMBY avoidance concerns.

      1. But the LM descent module engine was pressure fed and burned hypergolic propellants. That’s quite different from a kerosene/LOX engine that uses turbopumps when it comes to throttleability.

        1. There are differences, but the key thing for throttleability is to keep the pressure of the incoming propellants more or less constant while radically varying their total mass flow rate. The latter issue is addressable with variable valving. The former issue is addressable either through varying turbopump speed or, probably the better approach, a wastegate and recirculation propellant path to allow the pumps to run at constant speed. Current Merlins, being at least somewhat throttleable, already have some kind of variable valving. It just needs to be made more variable. Adding a wastegate and recirculation system if there isn’t already one incorporated in Merlin’s design would be a significant engineering modification. Not being privy to the details of the Merlin design, I have no insider scoop to offer here.

    4. I think you’re wrong about LEO re-entries being easier than those from GTO orbits. At GTO perigee, an S2’s speed should be essentially the same as that of an S2 that was in a lower orbit all along.

  7. They should really do land recovery of the second stage from the outset, preferably in some location such as Black Rock. Overflight of CONUS for an Atlantic recovery will be much more difficult from a regulatory standpoint.

  9. OK, so lets put a heat shield on the front of the second stage but make the stage “fluffy” by having conformal panels hinged at the bottom of the stage (engine end) open out for nose first re-entry. That ought to keep it pointed in the right direction and minimize heating loads and heat shield mass.
    When reaching lower atmosphere the terminal velocity will be low and fuel for soft powered landing minimal with say 3 Super Draco like rockets at top of stage.
    If you want, terminal velocity could be even lower by twisting the panels like autogyro blades (did someone say Rotary Rocket?).
    Untwist to stop spin just before rocket powered touchdown. Or power the tips with small solids or liquid mono propellant and twist the other way to act like helicopter in last few seconds. Not much fuel and no big rocket motors needed. Heat shielding requirements eased, panels can act as insulation on orbit to keep LOX cold for longer.
    Make heat shield at front split open like flower for landing legs. Will need small rocket for de-orbit/soft touchdown there anyway. Spin opposite to stage body for touchdown or mount aero panels on rotating ring around engine.

  10. I don’t see why landing S2 on the ASDS instead of land would be a problem, because they do have two. If it’s a launch from the Cape, land S1 on OCSLY, S2 on JRTI in the Pacific.

    My guess, though, is the first attempt will be more like we saw with payload shrouds and S1 on the early tries; water landing.

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